CyanogenMod is an “Android Community Rom based on the Donut tree”. It is, to quote some more, “focused on speed, followed by functionality from the core system (eg stuff that can’t be added on easily) and then stability.”
My experience is, that going from Cupcake (a.k.a Android 1.5) to this was as big of a speed boost as going from 1.0 to Cupcake was. This video shows the general speed of some core apps.
For the hackers out there: Check out Cyanogen on GitBug.
